MCCBs are employed to interrupt DC or AC, single-phase or multi-phase, electrical circuits for protection of the electrical infrastructure when an electrical fault condition occurs. The electrical fault conditions can include an instantaneous current in the circuit that exceeds a predefined instantaneous current limit (i.e., an electrical short exists) or a long-term current that exceeds a predefined long-term current limit (i.e., an overload condition exists). A single-break MCCB typically comprises one pair of electrical contacts for each phase, with each pair consisting of a stationary contact mounted to a stationary current loop and a movable contact mounted to a contact arm. A dual-break MCCB comprises two pairs of electrical contacts per phase. Per phase there are two stationary contacts and a contact arm with two movable contacts. A circuit breaker mechanism interrupts the flow of electrical current by separating the movable contacts from the stationary contacts, thereby transitioning from a closed to a tripped state.
In conventional approaches, the circuit breaker mechanism moves from the closed to the tripped state by releasing stored elastic strain energy from a helical tension spring and converting it to kinetic energy of the mechanism links. The release of elastic strain energy begins by disengaging a cradle from a latch assembly that helps hold the spring under tension in normal operation when the circuit breaker mechanism is closed. The latch assembly can be disengaged by an automatic trip unit that senses and responds to an electrical fault or manually by an operator pressing a “push-to-trip” button.
The energy to separate the movable contact and contact arm from the stationary contact may also come from an electromagnetic field that develops around the stationary current loop and the contact arm due to the short circuit currents that flow through these components. The interactions between the electromagnetic field and the short circuit current result in a repulsive force between the stationary current loop and the contact arm which causes them to move apart (i.e., “blow-open”). The magnitude of the repulsive force diminishes as the current and electromagnetic field intensity drop when the contacts start to separate. The circuit breaker mechanism has to be capable of preventing the contact arm from reclosing with the stationary contact.
Once tripped, the circuit breaker mechanism remains tripped and indicates this state to an operator. The circuit breaker mechanism is typically resettable after being tripped by moving a handle from the tripped position to the open (i.e., off) position. This increases the elastic strain energy of the spring and engages the cradle with the latch assembly. Once reset the electrical circuit can be closed by moving the handle from the open position to the closed (i.e., on) position. MCCBs may also be used to interrupt and close electrical circuits in absence of electrical faults by moving a handle between the closed and open positions.
The movement of the contact arm from the closed position to the tripped position should be fast to minimize the formation of arcs that may degrade the contacts and thereby increase the overall electrical resistance of the circuit breaker. Similarly, when a handle is used to transition the circuit breaker mechanism between the closed state and the open state, the contacts have to move quickly even if the handle motion is slow. This characteristic is called “quick make-break”. The circuit breaker mechanism has to be of suitable size to fit in a predefined circuit breaker casing or electrical panel. The circuit breaker mechanism should be insensitive to wear, contamination, long-term fatigue, vibrations, temperature and humidity to prevent unintended nuisance trips or a no-trip situation.
Due to these design criteria, conventional circuit breaker mechanisms have a variety of moving components. Correspondingly, they are difficult to assemble and have a high number of failure modes, sources for friction and other uncertainties affecting their performance characteristics. In addition, conventional circuit breaker mechanisms may be larger than desired and still not meet all of the design criteria with regard to the opening speed or repeatability of the displacement input or force input at which they can be tripped.
Therefore, there is a need for circuit breaker mechanisms that are less complex, have more repeatable performance characteristics, are easier to assemble, are smaller in size, have faster opening times and more repeatable displacement or force inputs at which they can be tripped.